Mercury exports from a High-Arctic river basin in Northeast Greenland (74°N) largely controlled by glacial lake outburst floods

Highlights

• Hg concentrations and exports in the Zackenberg River were assessed from 2009–2013.

• Hg in snow, soil and permafrost was measured within the river basin (ZRB).

• Hg yields from the ZRB were among the highest reported from Arctic river basins.

• Glacial lake outburst floods accounted for up to > 31% of the annual Hg export.

• Winter snowfall and summer temp. were also found to be important indirect controls.

Abstract

Riverine mercury (Hg) export dynamics from the Zackenberg River Basin (ZRB) in Northeast Greenland were studied for the period 2009–2013. Dissolved and sediment-bound Hg was measured regularly in the Zackenberg River throughout the periods with running water (June–October) and coupled to water discharge measurements. Also, a few samples of snow, soil, and permafrost were analysed for Hg. Mean concentrations of dissolved and sediment-bound Hg in the river water (± SD) were 0.39 ± 0.13 and 5.5 ± 1.4 ng L^− 1, respectively, and mean concentrations of Hg in the river sediment were 0.033 ± 0.025 mg kg^− 1. Temporal variations in river Hg were mainly associated with snowmelt, sudden erosion events, and outburst floods from a glacier-dammed lake in the upper part of the ZRB. Annual Hg exports from the 514 km² ZRB varied from 0.71 to > 1.57 kg and the majority (86–96%) was associated with sediment-bound Hg. Hg yields from the ZRB varied from 1.4–3.1 g Hg km^− 2 yr^− 1 and were among the highest yields reported from Arctic river basins. River exports of Hg from ZRB were found to be largely controlled by the frequency, magnitude and timing of the glacial lake outburst floods, which occurred in four of the five years in July–August. Floods accounted for 5 to > 10% of the annual water discharge, and up to > 31% of the annual Hg export. Also, the winter snowfall and the summer temperatures were found to be important indirect controls on the annual Hg export. The occurrence and timing of glacial lake outburst floods in the ZRB in late summer at the time of maximum soil thaw depth, the location of the glacier in the upper ZRB, and increased thawing of the permafrost in Zackenberg in recent years leading to destabilisation of river banks are considered central factors explaining the high fraction of flood-controlled Hg export in this area.

Introduction

Mercury (Hg) is a well-known contaminant of concern to top predators and the human population in the Arctic (AMAP, 2011). Riverine exports of Hg from river basins to the ocean constitutes an important part of the Arctic Hg cycle, but relatively few studies have attempted to quantify the sizes and controls of Arctic river exports. Previous studies that have examined Hg exports from Arctic rivers include studies from Siberia (Coquery et al., 1995), Canada (Semkin et al., 2005, Leitch et al., 2007, Hare et al., 2008, Kirk and St. Louis, 2009, Schuster et al., 2011, Emmerton et al., 2013) and Greenland (Rigét et al., 2011, Søndergaard et al., 2012). Sources of Hg from river basins in the Arctic include Hg from recent atmospheric deposition, and Hg stored in soil, permafrost and glacier ice (Douglas et al., 2012, Stern et al., 2012, Zdanowics et al., 2013). The contribution of Hg from these sources may be significantly influenced by climate variability and change, and there is a concern that warming in the Arctic may lead to increased future mobilization and riverine export of Hg to the marine environment (Schuster et al., 2011, Fisher et al., 2012, Stern et al., 2012). To increase the knowledge on the controls of Arctic riverine Hg export, and to establish a baseline that can be used for comparison with future conditions, there is a need for more studies on Hg export dynamics from Arctic river basins.

Thus, the aim of this study was to investigate the size and controls of the riverine Hg exports and yields from the Zackenberg River Basin (ZRB) in Northeast Greenland (74° N). The ZRB represents a relatively small river basin of 514 km² and is not influenced by ground water. Therefore, Hg released from snow, ice, soil or rain to water within the river basin was expected to be measured at a measuring station near the river mouth within a relatively short time. This makes ZRB ideal for studying small-scale processes and controls for riverine Hg exports. Glacial outburst floods have regularly been observed in Zackenberg River since the Zackenberg Research Station was established in 1996. The overall hypothesis of the study was that the frequency, magnitude, and timing of these floods and variations in meteorological variables such as temperature and precipitation were likely to have a high impact on the riverine Hg budget. To examine this, high-time-resolution water samples were taken from the Zackenberg River during 5 years from 2009 to 2013 throughout the periods with running water and coupled to continuous water discharge measurements. Both dissolved Hg and sediment-bound Hg were measured in the water samples. Data on air temperature, snow depth, and precipitation from the Zackenberg Research Station were used to examine controls. In addition to the above, a limited amount of snow samples from the ZRB was collected during spring to evaluate atmospheric deposition of Hg to snow, and snow as a source of riverine Hg. Also, a limited amount of soil and permafrost samples was analysed for total Hg to assess concentrations. Finally, data from the Zackenberg River was compared against published data from a range of other Arctic river studies. A minor part of the data (river data from 2009) was previously published in Rigét et al. (2011).